Free-field noise sources, such as internal combustion engines and combustion turbines, generate powerful low-frequency noise in the 31 Hz and 63 Hz octave bands (where the 31 Hz octave band extends from 22 Hz to 44 Hz and the 63 Hz octave band extends from 44 Hz to 88 Hz). Passive noise control requires the use of large, expensive silencers to absorb and block the noise. The size and cost of such silencers makes passive control unacceptable for many applications. An alternative to passive control is a combination of passive control and active control. Passive control abates noise better as the frequency of the noise increases and active control works better as the frequency of the noise decreases. Therefore a combination of passive and active control may advantageously be employed in many applications.
The active control of sound or vibration involves the introduction of a number of controlled "secondary" sources driven such that the field of acoustic waves generated by these sources destructively interferes with the field generated by the original "primary" source. The extent to which such destructive interference is possible depends on the geometric arrangement of the primary and secondary sources and on the spectrum of the field produced by the primary source. Considerable cancellation of the primary field can be achieved if the primary and secondary sources are positioned within a half-wavelength of each other at the frequency of interest.
One form of primary field that is of particular practical importance is that produced by rotating or reciprocating machines. The waveform of the primary field generated by these machines is nearly periodic and, since it is generally possible to directly observe the action of the machine producing the original disturbance, the fundamental frequency of the excitation is generally known. Each secondary source can therefore be driven at a harmonic of the fundamental frequency by a controller that adjusts the amplitude and phase of a reference signal and uses the resulting "filtered" reference signal to drive the secondary source. In addition, it is often desirable to make this controller adaptive, since the frequency and/or spatial distribution of the primary field may change with time and the controller must track this change.
To construct a practical adaptive controller, a measurable error parameter must be defined and the controller must be capable of minimizing this parameter. One error parameter that can be directly measured is the sum of the squares of the outputs of a number of sensors. The signal processing problem in a system employing such an error parameter is to design an adaptive algorithm to minimize the sum of the squares of a number of sensor outputs by adjusting the magnitude and phases of the sinusoidal inputs to a number of secondary sources. S. J. Elliot et al., in "A Multiple Error LMS Algorithm and Its Application to Active Control of Sound and Vibration," IEEE Trans. on Acoustics, Speech and Signal Processing, Vol. ASSP-35, No. 10, October 1987, describe a least-mean-squares (LMS) based active noise control system, however that system converges too slowly for many applications.
The present invention is directed to systems for controlling both random and periodic noise in a single or multiple mode acoustic environment. (In a multiple mode acoustic environment the amplitude of the sound varies in a plane perpendicular to the direction in which the sound propagates.) There are known systems for controlling random noise propagating in a single mode through a duct, however these systems do not work with multiple mode propagation. See U.S. Pat. Nos. 4,044,203, 4,637,048 and 4,665,549 and M. A. Swinbanks, "The Active Control of Low Frequency Sound in a Gas Turbine Compressor Installation." Inter-Noise 1982, San Francisco, Calif. May 17-19, 1982. pp. 423-427 .